The present invention relates generally to optoelectronic devices for use in optical communication systems. More specifically, the present invention relates to an improved mirror structure with integrated optical feedback protection for use in a vertical cavity surface emitting laser (VCSEL), such as is typically used as an optical transmitter in optical communication systems.
It is known in the art that VCSEL devices have heightened sensitivity to some forms of optical feedback. In this regard, for the context of the present application, we define optical feedback as any light incident on the VCSEL output mirror from the outside, traveling back into the VCSEL. Initially, one would expect that since the output mirror of the VCSEL is highly reflective in both directions, usually between 99% and 99.9%, very little incident light, on the order of 1% to 0.1%, actually penetrates the mirror over the wide range of wavelengths that the mirror is highly reflective. However, the VCSEL is a resonant cavity structure that includes a volume of material sandwiched between the output or front mirror and a backside mirror, both of which are parallel highly reflective mirrors. FIG. 1a shows a graph of the reflectivity of a symmetric lossless cavity for light incident on the front mirror from the outside, where the two mirrors have identical reflectivities of about 99.5% and where lossless means no parasitic losses other than the light exiting through the two mirrors. As can be seen, the reflectivity of the whole structure is very robust at approximately 99.8% or higher over a wavelength range of almost 300 nm. The difficulty however is that for a very narrow range that exists at a fraction of a nanometer, the 0.5% portion of the incident light that does penetrate the output mirror is amplified inside the cavity due to resonance. The center of that range is called the resonant cavity wavelength (rcw). The 0.5% of the incident photons that penetrate the front mirror from the outside begin to bounce back and forth between the two mirrors in an amplification process of trapping. Only 0.5% of the incident photons that entered the cavity from the outside can exit through each mirror per pass, defined as one round trip inside the cavity. Meantime, during each pass, more externally incident light penetrates the front mirror that is also mostly trapped. This buildup of trapped incident photons continues inside the cavity until the amount leaving the cavity equals the amount entering the cavity. The amplification builds up until the light exiting through the output mirror from the cavity, being out of phase with the 99.5% reflected portion of the incident light, interferes destructively with it and completely cancels the reflection. This is seen in FIG. 1a and the expanded view of FIG. 1b as a narrow notch in the reflectivity, wherein the reflectivity of the mirror structure can be seen as going down to 0% at rcw. At the same time, the light exiting the back mirror, that is, the light transmitted through the cavity, has the intensity of the incident light. Thus the symmetric cavity structure appears to be transparent to incident light at rcw. The total cancellation of the reflection and the total transmission through the cavity, at rcw, are not the critical facts, as these are just symptomatic consequences of the more important fact that inside the cavity, the light is at least two orders of magnitude brighter at rcw than the incident light due to resonance. This means that that externally incident light on the front mirror of the symmetric cavity, at rcw, produces light inside the cavity that is about 4 orders of magnitude brighter than the light that penetrates a stand alone front mirror, per pass, or the incident light inside the mirror at a wavelength outside of the narrow notch of FIGS. 1a and 1b. Such amplified light represents a significant disturbance inside the cavity of this structure.
While comparable to the above structure, a VCSEL is different in some aspects in that it is not a symmetric cavity and it has some parasitic losses and gain. As a result, one usually will not see such manifestations of resonance for a VCSEL as is shown in FIGS. 1a and 1b. There is usually hardly any apparent transparency and the reflectivity notch is much shallower, because the back mirror reflectivity is commonly greater than the front mirror reflectivity in a VCSEL. Nevertheless, the same principles apply in that amplification usually greater than a factor of 100 at and near rcw still occurs in a VCSEL, and could be nearly a factor of 1000. As the amplification increases, the range of wavelengths over which amplification occurs (the width of the notch in FIGS. 1a and 1b) decreases. In VCSELs that notch width could be a fraction of an Angstrom. As a result, the VCSEL is essentially impervious to optical feedback except at rcw and the narrow range of wavelengths within its notch.
The difficulty arises in VCSELs, however, because in optical communication systems the most likely and perhaps the only forms of optical feedback that occur are back reflections of the VCSEL's own output, which by its very nature is at rcw. Thus a VCSEL is mostly vulnerable to feedback in the form of its own reflections and must be protected from them, despite the high reflectivity of the front mirror.
Accordingly, in fiber optic communication system applications, there are a variety of ways in which such back reflection of laser output may be generated creating the potential for interference with the operation of the VCSEL. In a typical system, the transmission signal is generated and/or received by an optical subassembly that is constructed specifically for this purpose. The optical subassembly may typically contain a light-generating device such as a semiconductor laser (a VCSEL, for instance) for transmitting a signal, a photodiode for the purpose of receiving a signal or both, should the subassembly be configured as a transceiver that both transmits and receives optical communications signals. In such fiber optic communication systems and certain other applications, this optical subassembly is also configured to couple the light that is generated by the semiconductor laser into an end face of an optical fiber. The far end of the fiber is then coupled to another transceiver or a receiver. The fiber may also have passive connectors or splices between the two subassemblies. It is in all of these couplings that reflections, which may be produced by any number of components within the optical sub-assemblies or in the fiber (such as the fiber end faces, optical lens elements, beamsplitters, polarizers optical isolators, or the connectors) may result in feedback to the laser, in the form of back reflections of the laser's own output. This is problematic because semiconductor lasers, including VCSELs, are very sensitive to this type of optical feedback, as was explained above, and their performance may be dramatically impacted by such reflection-produced feedback. For example, it is known that medium to strong feedback may give rise to relative intensity noise (RIN), power modulation or other coupled cavity effects. In general, one of the most sensitive and troublesome indicators of a back reflection problem is the increase in RIN. Thus fiber optic communication system specifications usually include the requirement that RIN should not increase appreciably in transmitter experiments where a −12 dB (˜6.3%) back reflection is intentionally produced.
At this point, the discussion of back reflection effects in VCSELs needs to be divided into two separate discussions covering two different regimes. The first is where the surfaces creating back reflections are located within the coherence length of the VCSEL, and the second is where the reflecting surfaces are located beyond the coherence length. The main significance of the coherence length, L, is that interference effects occur at distances shorter than L and no longer do so at distances greater than L. The coherence length can be determined by the following equation.
  L  =                    λ        2                    n        ⁢                                  ⁢        Δ        ⁢                                  ⁢        λ              .  
Here λ is the rcw, Δλ is the width of the resonance notch described above, and n is the index of refraction of the medium in which the light is traveling, usually optical fiber in communication systems, where n≈1.5. For VCSELs, L is typically between 1 and 6 cm. Interference occurs when the lightwaves are mostly in phase with each other and stops when their phases become random. Obviously, L is not a sharp cutoff, but it so happens that the potentially reflecting surfaces inside the transceiver package or optical subassembly are usually at a distance from the VCSEL that is much shorter than L, and the potentially reflecting surfaces along the fiber and at the other end of the fiber are usually at a distance from the VCSEL that is much longer than L. This is a critical fact for two reasons. The first is that the design of the package is under the control of the transceiver manufacturer, but what happens in the fiber and on the other end of the fiber probably is not. The second critical fact is that the protection from optical back reflections proposed in this patent will work when phases are randomized, but may not work when the lightwaves are in phase, for reasons that will become apparent in the description that will follow. Thus the proposed protection in this patent will work for those back reflections, which the manufacturer cannot otherwise prevent without costly isolators, and may not work only for back reflections from inside the package, in which the manufacturer can incorporate additional inexpensive protection. For example, if the communications system utilizes multimode fiber, as is the case with applications using short-wave VCSELs (850 nm, for instance) and in some cases using long-wave VCSELs (1.3 μm, for instance), the coupling of the laser light into the fiber can be so configured as to couple very little of the distant back reflections, returning through the fiber back into the laser. In this regard, in the case of multimode fiber systems, building protection from feedback into the VCSEL is less critical. However, in single mode fiber systems, the coupling is essentially symmetric and since it is designed to maximize the coupling of laser light into the fiber, it tends to maximize the coupling of light from the fiber back into the laser. The only alternative solution is the much more expensive optical isolator, used with edge-emitting lasers, which requires the laser to emit light with a controlled polarization. Controlling the polarization of a VCSEL output can have performance and reliability costs, which coupled with the monetary costs of the isolator, makes the VCSEL requiring an isolator a less convincing alternative to edge-emitting lasers.
Usually, VCSEL mirrors, especially the output mirrors, are Distributed Bragg Reflectors (DBRs), comprising stacks of layers with alternating high and low indices of refraction. Each interface in the stack produces a small reflection due to the change in the index of refraction. When each layer is made a quarter of the rcw thick, all the reflections, being in phase, interfere constructively, to produce a high reflectivity, peaking at rcw. When the number of layers is large enough, the DBR reflectivity can become as large as 99% or even 99.99% at rcw, provided that there is very little to no absorption or scattering loss of light in any of the layers or interfaces. In the case of no loss, the light that is not reflected, is transmitted, that is, penetrates the mirror. In addition, as the change in index between alternating layers become larger, the range of wavelengths becomes larger, over which the reflectivity is essentially as large as it is at rcw (over hundreds of nm, for the exemplary mirror used for FIGS. 1a and 1b). Lossless DBRs are symmetric, in the sense that they have the same reflectivity, R, and transmissivity, T, whether the light is incident from one side or the other, and for both directions of incidence R+T=1, as long as the mirror remains lossless. Then, if there are some absorption losses, A, as long as they are more or less uniformly distributed, the mirror remains essentially symmetric, in the above sense, and then R+T+A=1 for both directions of incidence. However, if the absorption is all concentrated near one face, then the amount of light lost to absorption is much greater on that side than on the other, which means that R+T on the opposite side becomes much greater than on the absorber side, breaking the above-mentioned DBR symmetry. This phenomenon is the underlying principle of the invention proposed herein.
Referring to FIG. 2, it can be seen that prior art attempts were made to reduce the external-side reflectivity of the emitting mirror 18 of a VCSEL 10 by integrating an absorptive layer 28 in order to reduce the reflectivity as seen by a feedback optical wave. Typically, the VCSEL 10 included a lower mirror 14 formed above a substrate 12, an optical cavity 16 formed above the lower mirror and an upper mirror 18 formed above the optical cavity 16. The upper mirror 18 of this device was a hybrid mirror, having a semiconductor portion 20 and a dielectric portion 22. The device further included a current confinement implant 24 as well as a current constriction 26 for mode control and defining individual devices on a wafer.
The dielectric portion 22 of the hybrid mirror comprised alternating one-quarter wavelength thick layers of a high index of refraction dielectric material and a low index of refraction dielectric material. In this approach, an absorptive titanium layer 28 was formed at the low-to-high index transition closest to the emitting facet. In this embodiment, the titanium layer 28 was processed to remove it from within the aperture formed by an upper ohmic contact 30 to reduce the absorption losses as seen from the cavity. The difficulty encountered with this approach, however, is that it provides little to no absorption of the optical feedback as seen from the external cavity. In particular, the large number of longitudinal modes that appear in the transmission spectrum due to the external cavity is not reduced.
In an attempt to overcome this absorption loss problem, U.S. Pat. No. 6,882,673, the entire contents of which are incorporated herein by reference, discloses that if the absorption layer is made very highly absorbing, but very thin, it can be placed in the DBR structure, not only near the medium and far from the cavity, but also at a null of the cavity standing wave, which means that very little of the internal laser light, will actually encounter the absorption. Its absorption contribution will then be made even smaller, but the absorption of the fed back light will not be affected as long as it is incoherent, since that light is then not in the form of a standing wave and has no positional null. As is shown in FIG. 3, an absorption layer 28a is provided in between a pair of mirror layers in the emitting mirror 22a and ultimately comprises three layers that are incorporated into the VCSEL 10 structure in place of the last mirror pair.
While this prior art approach has been effective in reducing the amount of feedback that enters the optical cavity, this solution also has difficulties. In particular, what is not provided in the prior art is a method of providing a laser with a mirror structure that produces an η (the laser's LI slope), as large as the one obtained with an emitting mirror without the feedback protection. As a result, it is possible that while a VCSEL may include the desired feedback protection, it may not be able to meet other device performance requirements, such as specifications set on η, output power, threshold current, etc.
There is therefore a need for a unique VCSEL construction that includes feedback protection while also allowing the VCSEL structure to be tuned to accommodate specifications having a set η, a particular power output requirement or a particular threshold current.